[bigint] Fix mjsunit pretty-printing of Numbers

Implicit case-fallthrough strikes again.

Bug: v8:6791
Change-Id: Iee6422a67797f8958527507bac538bcdac2ebddc
Reviewed-on: https://chromium-review.googlesource.com/747075Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49057}

test/mjsunit/mjsunit.js

@@ -228,11 +228,11 @@ var failWithMessage;
     switch (typeof value) {
       case "string":
         return JSON.stringify(value);
+      case "bigint":
+        return String(value) + "n";
       case "number":
         if (value === 0 && (1 / value) < 0) return "-0";
         // FALLTHROUGH.
-      case "bigint":
-        return String(value) + "n";
       case "boolean":
       case "undefined":
       case "function":